An A/F sensor for vehicle is used for detecting a combustion condition in an internal combustion engine by detecting an oxygen concentration in a measurement gas (exhaust gas).
As an example for automobiles, an A/F sensor is a product that detects an oxygen concentration in a measurement gas (exhaust gas) using an electromotive force as limit current generated in a solid electrolyte of the A/F sensor element due to an oxygen concentration difference between a reference gas and the measurement gas (exhaust gas) as an output.
An oxygen sensor element of one cell type is widely used for the A/F sensor element.
Generally, the A/F sensor element is composed of a solid electrolyte such as zirconium oxide partially stabilized with yttria, and a pair of platinum electrodes provided on both surfaces of the solid electrolyte.
A surface which is exposed to the exhaust gas of the electrode among the pair of electrodes of the A/F sensor element is provided with a diffusion resistance layer made of a porous ceramic.
The diffusion resistance layer can allow the measurement gas such as the exhaust gas to permeate therethrough, thus the measurement gas is introduced into the element via through holes of the diffusion resistance layer, and reaches the electrode formed on the solid electrolyte.
Since it is necessary to partition spatially the exhaust gas and atmosphere, which is a reference oxygen concentration, by the solid electrolyte in the A/F sensor element, an A/F sensor element having a bottomed cylindrical shape or a plate shape is used.
Since the plate-shaped A/F sensor element plate can be manufactured by laminating sheets of solid electrolyte layers or insulating layers, it is easy to manufacture.
Further, since it becomes possible to laminate-form a heater integrally with the solid electrolyte layers for heating the element, it is easy to heat the solid electrolyte layer.
However, due to its plate-like overall shape, corners are formed at ends, and the element is poor at handling thermal shock in a usage environment or when being covered by water in an exhaust pipe, so that there is a possibility that the element may be damaged.
On the other hand, since a bottom can be formed in a curved surface in the bottomed-cylindrical-shaped A/F sensor element, thermal shock is dispersed, thus it is advantageous that cracks due to the water or the like can be prevent from occurring.
An element made entirely of a solid electrolyte such as zirconia device as the A/F sensor element having the bottomed cylindrical shape has been developed, for example (refer to Japanese Patent Application Laid-Open Publication No. 53-139595).
However, zirconia has low thermal conductivity.
Therefore, if a whole A/F sensor element is formed by zirconia, the time it takes to heat the element sufficiently becomes longer when heating the element by a heater inserted and disposed in the element having the bottomed cylindrical shape.
As a result, there is a problem that a quick activation of the A/F sensor element cannot be performed.
Further, partially stabilized zirconia in which expensive rare earths such as yttria is added to zirconia is used as the solid electrolyte in recent years.
However, an amount of rare earth increases if entire element is formed by the solid electrolyte made of partially stabilized zirconia as in the conventional art, manufacturing cost increases.